1. Field of the Invention
The present invention relates to a mobile communication system, and particularly, to an apparatus and method for sending an emergency rescue signal using a mobile communication terminal.
2. Description of the Background Art
Generally, there are various methods for estimating a position of a terminal in a mobile communication system. Among those methods, a method which can be applied to a cellular system based on CDMA (Code Division Multiple Access) can be classified into a method which needs to modify a terminal and a method which does not need to modify a terminal.
First, a representative method in the above method which needs to modify the terminal is a plotting method based on a terminal, and this method requires a GPS (Global Positioning System) receiver to be installed in a mobile communication terminal carried by a user.
In case a user of the terminal is under an emergency condition, when the GPS receiver is operated by pushing a predetermined button (for instance, a rescue requesting button) on the mobile communication terminal in order to transmit position information of the mobile communication terminal, the GPS receiver installed in the mobile communication terminal receives a signal from three or more GPS satellites (four or more satellites, in case of estimating a position on a three dimension) to calculate distances between the mobile communication terminal and respective satellites and estimates the position of the mobile communication terminal using additional information received from the GPS satellite.
After that, the calculated position is automatically transmitted to a position that needs the position information of the user using the CDMA cellular network.
On the other hand, there are many methods which do not need to modify the terminal and a TOA (Time of Arrival) method using arrival time, an AOA (Angle of Arrival) method using arrival angles and a TDOA (Time Difference of Arrival) method using arrival time differences are commonly used.
First, with the TOA method using arrival time, a plurality of base stations around a mobile communication terminal receive a signal transmitted from the mobile communication terminal, and then, the respective base station measures the arrival time of the transmitted signal to calculate distances between the mobile communication terminal and the base station. And a virtual circle is made centering around the base station as making the distances as the radiuses, and then, a position where the circles are crossed to each other are estimated as the position of the mobile communication terminal.
Then, with the AOA method using an arrival angle, a plurality of base stations around a mobile communication terminal receive a signal transmitted from the mobile communication terminal, and the respective base station measures the direction of the transmitted signal. In addition, a virtual line toward the above direction is drawn, and a position where the lines are crossed with each other is estimated as the position of the mobile communication terminal.
With the TDOA method using arrival time difference, a plurality of base stations around a mobile communication terminal receive a signal transmitted from the mobile communication terminal. In addition, a hyperbolic curve is drawn using the arrival time differences of the signal transmitted to the respective base stations and a position where the hyperbolic curves are crossed to each other is estimated as the position of the mobile communication terminal.
However, according to the position estimating method using the GPS, the GPS receiver should be installed in the terminal additionally, a size and a weight of the terminal are increased. In addition, the GPS receiver also uses the power of a battery for the terminal, the power consumption of the terminal is increased.
Also, with the position estimating method using the GPS, the signal transmitted from the GPS satellite is very weakened when the signal is arrived at the terminal on the earth, and therefore it is difficult to receive the GPS signal in case that there are obstacles between the terminal and the GPS satellite such as in an urban area or indoor, whereby the function is lowered.
On the other hand, the TOA method using the arrival time and the TDOA method using the arrival time difference should use at least three base stations around the mobile communication terminal, and the AOA method using the arrival angle should also use at least two base stations. However, if the user is in a suburban area, there is little chance that there are two or three base stations around the user. Therefore, the signal from the mobile communication terminal is difficult to reach the other base stations, and even if the signal arrives at the other base stations, the signal is very weak and there may be a lot of error. Therefore, it is difficult to estimate the position of the mobile communication terminal.
As described above, it is difficult to estimate the position of the mobile communication terminal using only one estimating method using such the arrival time, arrival angle, or arrival time difference of signal in the suburban area.
Also, with the above conventional method, a position can be estimated approximately, but much time is required to estimate the position exactly. Accordingly, a new method for detecting an exact position of a rescue requester is required, under the condition that rapid and exact position detection is required, as in the emergency rescue.